A communication network comprises a plurality of (i.e. at least two) participants which communicate with one another via suitable communication channels. The communication channels can be designed as wireless and wired channels. Information can be transmitted in the communication network from one participant to another participant in the form of discrete data packets (messages).
A transmission network is a network in which a data path passing through the participants is defined, via which the participants can receive and forward messages. A message transmitted by a controller as a first participant in the network passes through the data path and therefore the subsequent participants of the transmission network. While the message passes through one of the participants, the participant concerned has the facility to read data from the message or write data to the message. A message may contain one or more datagrams. Each datagram comprises at least a header and a data field following the header. The header is provided to accommodate status information and/or control information. The data field is provided to accommodate payload.
In a write procedure, a participant receives a datagram and defines input data depending on the content of the header of the datagram. The participant then writes the input data to the data field following the header. The definition of the input data depending on the content of the header takes a certain amount of time. This time is referred to as the response time of the participant concerned.
Since the length of the datagram is fixed, a time interval, the length of which is similarly fixed, i.e. may not be modified by the participant, occurs between the output of the datagram header and the output of the data field of the datagram (in each case at the output of the participant). In other words, the time of the output of the header is linked in a fixed manner to the time of the output of the data field. However, the participant cannot output the data field until it has defined the aforementioned input data and has written it to the data field. Since a certain amount of time (the aforementioned response time) is used for the definition of the input data, the participant cannot output the header at any random early time. Instead, it delays the output of the entire datagram by a time corresponding to the response time (otherwise the header and data field would be “torn apart”).
In automation technology, different fieldbus protocols based on Ethernet at 100 Mbit/s have become established. Since Ethernet at 1 Gbit/s has now become prevalent in the consumer domain, it is desirable to transmit fieldbus protocols also at bit rates of 1 Gbit/s or more. At the same time, it is desirable not to have to clock the entire participant accordingly faster.
DE 198 03 686 A1 discloses a clocked fieldbus system with at least two participants which exchange messages bit-synchronously, wherein the processing time by the participants is taken into account by means of empty messages. Intermediate fields in messages which serve to fill the free space in payload fields which are not occupied with payload are known from DE 10 2007 004 044 A1 and DE 10 2008 009 835 A1.